This invention relates generally to flap doors, particularly those rotatable about horizontal hinge lines, and to hollow structures. More particularly, the invention relates to a combination of upper and lower flap doors for closing and opening a large opening in the wall of a structure such as a vehicle body, which doors are intercoupled by a mechanism in a manner such that torques due to gravity acting on the flap doors substantially counterbalance each other, whereby opening and closing manipulation of the flap doors is greatly facilitated and made smooth.
Heretofore, flap doors of the above mentioned character such as side boards of truck bodies have generally been opened and closed by hand in most cases. Where only a single flap door of relative small width (in the vertical direction perpendicular to the hinge line) is involved for each wall as in the case of a compact or medium sized truck, the manual operation thereof is simple and does not require much effort. However, in the case where a flap door is of considerable width, or where two flap doors, one above the other, are used, as in a large closed van of high ceiling whose side wall must be opened, the manual handling of the wide door or the two doors requires considerable physical strength and labor. Furthermore, there may be some danger of injury to the operator due to mishandling. In such a case, the door or doors can be power driven by means such as hydraulic cylinders, but such devices are expensive.